The video later shows the bodies of dozens of men stripped to their underwear and lying face down on the floor.

"The 250 shabeeha taken captive by the Islamic State [ISIS] from Tabqa in Raqqa have been executed," a caption posted with the video read. A separate pile of bodies can also be seen close by.

The group also released photos purporting to show captured soldiers.

Isis, a radical offshoot of al Qaeda, stormed the Tabqa airbase on Sunday after days of clashes with the army and said it had captured and killed soldiers and officers in one of the bloodiest confrontations yet between the two sides.

The Syrian Observatory said 346 Isis fighters and more than 170 members of the security forces were killed during the battle.

About 1,400 people are believed to have been station at the air base. Seven hundred are thought to have escaped